Normal fine paper contains internally some filler up to a maximum of about 30% mineral filler. As fine paper suitable for offset and gravure printing must have sufficient strength to resist the printing operation which is carried out under high speed, and this includes both tensile and Z-direction strength, it has been found that the use of high quantities of mineral filler are not suitable. Indeed, the normal offset printable fine paper has a very low mineral filler content, and this paper is normally surface sized after the paper web has been dried. The term "fine" paper is used in the conventional industry sense and includes tablet, bond, offset, coated printing papers, text and cover stock, coated publication paper, book paper and cotton paper; it does not include so-called "high-strength" paper products.
The use of filler internally in the manufacture of paper in general and fine paper in particular has been practiced for many years using common fillers such as kaolin clay, talc, titanium dioxide, calcium carbonate, hydrated aluminum silicate, diatomaceous earth and other insoluble inorganic compounds. The use of filler accomplishes two objectives: one is the extension of the paper-making fibers to reduce cost and the other is to obtain certain optical and physical properties such as brightness and opacity. In fine paper manufacture, fillers are normally added at a level of 4-20% by weight of the finished paper, although rarely as much as 30% filler has been used in Europe and 25% in the United States. Fine paper manufacture in part depends on hydrogen bonding and one problem which occurs in the use of more than 20% filler in fine paper manufacture is that too much filler reduces hydrogen bonding and causes the web to lose its strength. Using external methods of application, such as coating with pigment/adhesive mixture on the size press or coater, the total filler content can easily be increased.
Fine paper containing up to a maximum of 30% filler is normally made by adding 15-20 pounds of cationic starch or 1-5 pounds of guar gum per ton of dry furnish, as normal internal strength agents. Latices are sometimes used in paper manufacture as noted below, but not in fine paper manufacture because such latices are normally sticky and difficult to use on a Fourdrinier machine for making fine paper at high speed.
The U.S. Pat. No. 3,184,373 to Arledter discloses the production of paper having greater than normal quantities of mineral fiber, but no mention is made of the properties of the resultant paper. The Arledter process depends on what is referred to as a synergistic mixture of filler retention aids, including a water soluble mucilaginous material, such as guar gum, and a water-soluble polyethylene imine resin. An earlier patent in the name of the same patentee, U.S. Pat. No. 2,943,013, contains similar subject matter, but the resultant paper is specified to be for use in the manufacture of decorative laminates, i.e. there is no requirement for the high strength necessary for fine papers which are to be printed by the offset method.
It has been common knowledge in the paper industry that the addition of an anionic latex to the wet end of a paper machine combined with a cationically charged chemical, such as alum, causes the latex to precipitate in the presence of the paper-making fibers and fillers and thereby gives the paper increased strength. This procedure is normally used in the manufacture of certain so-called "high-strength" products such as gasket material, saturated paperboard, roofing felt, flooring felt, etc. No similar technique has heretofore been suggested for the manufacture of fine paper having greater than normal quantities of mineral filler.
A number of prior patents disclose the general idea that a charged latex can be added to the paper-making furnish. Because of the basic electro-chemical reaction of an anionic paper-making system, a cationic latex precipitates easily and provides additional fiber bonding and, accordingly, strength to the resultant paper. These patents relate primarily to so-called "high-strength" papers which are largely devoid of fillers, or at best contain only very small quantities of fillers or pigments. For example, Wessling et al U.S. Pat. No. 4,178,205 discusses the use of a cationic latex, but pigment is not essential. Also the U.S. Pat. No. 4,187,142 to Pickleman et al discloses the use of an anionic polymer co-additive with a cationic latex, with the use of a sufficient amount of latex to make the entire paper-making system cationic; the use of fillers in any example is not mentioned. Foster et al U.S. Pat. No. 4,189,345 discusses extremely high levels of cationic latex.
It has been proposed noting the McReynolds U.S. Pat. No. 4,225,383, in the manufacture of relatively thick paper product, similarly to the manufacture of roofing and flooring felt papers, to use the combination of a cationic polymer with an anionic latex, and substantial quantities of mineral filler. Once again, however, the product is not designed for printing using the offset method, and its strength requirements are accordingly relatively low. Moreover, because of the substantial thickness of the products produced by such a technique, the product is given some additional strength merely by means of its mass.
The Riddell et al U.S. Pat. No. 4,181,567 is directed to the manufacture of paper using an agglomerate of ionic polymer and relatively large quantities of filler. The patentees indicate that either anionic or cationic polymers may be used, and fillers mentioned are calcium carbonate, clay, talc, titanium dioxide and mixtures. In example 1, an 80 basis weight paper having 29% ash is produced using calcium carbonate as the filler. This patent in essence discusses precipitation of the pigment with a retention aid system prior to its addition to the paper-making system.
Such Riddell et al patent mentions German Offenlegunschrift No. 25 16 097 near the bottom of column 1 thereof, the latter of which corresponds to U.K. Pat. No. 1,505,641 which discloses the pre-treatment of calcium carbonate with a styrene-butadiene latex to produce a protected pigment which can then be used in paper making preferably at the 20% by weight level, although the patent does state that there is little or no reduction in strength up to the 50% by weight level. In more detail, the U.K. patent discloses mixing an anionic latex with an aqueous suspension of the special filler having a cationic charge, e.g. made by mixing with positively charged starch. One to twenty parts of latex are used per 100 parts of filler, and the filler composition is added to the beater, pulper or elsewhere before the breast box. Example III shows the use of 400 parts of filler to 700 parts of wood fiber. A point to be emphasized, however, is that the technique of the U.K. patent requires extra equipment and extra processing, as the filler is first encapsulated and then only later added to the paper-making system; in other words, the technique of the U.K. patent is unduly complex. Moreover, the encapsulation provides inadequate protection to enable the calcium carbonate to be used in acidic medium without undesirable foaming.